In order to attain a stable operation of an apparatus mounted with ICs and a microcomputer, a direct current (DC) voltage is necessary which is stabilized to prevent any voltage drift. Recent years, in most of the electronic apparatuses, a switching regulator is adopted for supplying the stabilized DC voltage.
The switching regulator has features which are small in size and light in weight and functions as a DC-DC converter with good efficiency. Therefore, the switching regulator is widely used as a power supply for a microcomputer and a personal computer which are incorporated into various equipments. In these personal computers, a power consumption amount in a high-speed operation tends to increase. Also, the power consumption amount increases and decreases abruptly according to a processing load in the personal computer.
Also, in one feature, the switching regulator is possible to operate in a wide input voltage range. Therefore, the switching regulator is used as a power supply, which is operable in both of the area of 100V and the area of 200V in household voltage, and in which a specification range of input voltage is wide.
The switching regulator must stably control an output voltage to a target voltage according to change of the load of the personal computer. Moreover, in the switching regulator, even when the output voltage makes a transient response to a rapid change of load current and input voltage, it is required to quickly recover to the stable state of output voltage.
As methods of controlling the switching regulator, there are a voltage mode control and a current mode control. In the voltage mode control, switching is executed based on a pulse control signal generated through comparison of a triangle wave signal and an error control signal which is equivalent to a difference between the output voltage and a reference voltage, in order to stabilize output voltage. In the current mode control, the switching is executed based on an inductor current in addition to the error control signal, in order to further stabilize the output voltage.
In the voltage mode control, a phase margin must be secured at a point that the phase characteristic changes rapidly near a resonant frequency of an inductor and an output capacitor. Therefore, there is a problem that the power supply characteristic becomes unstable. On the other hand, in the current mode control, because a feedback circuit for controlling the inductor current and a current loop circuit which contains the inductor operate as an equivalent current source, the inductor characteristic does not have an influence on the power supply characteristic such as the frequency characteristic. As a result, the larger phase margin can be obtained in the current mode control, compared with the voltage mode control. Moreover, in the current mode control, from the above-mentioned reasons, it is possible to attain the current mode control in a wide bandwidth while maintaining the phase margin by extending a frequency band of the feedback circuit. Therefore, in the current mode control, the response characteristic to a load change can be improved compared with the voltage mode control.
A control system which executes the current mode control contains a voltage feedback system for stabilizing an output voltage and a current feedback system for controlling an inductor current. Therefore, it is necessary to provide the measure to restrain an influence by external noise and switching noise in each feedback system. Also, in the current feedback system of an analog circuit, a current detection resistance, an I-V converting circuit, and a slope compensation circuit need to be provided for the current mode control. Here, the current detection resistance and the I-V converting circuit are used to detect the inductor current. Therefore, the control system which executes the current mode control is larger in circuit scale than the control system which executes the voltage mode control, and it requires a complicated timing control. Therefore, in the switching regulator which realizes the current mode control, a peripheral circuit becomes large, and the number of parts on a printed circuit board and the mounting cost sometimes increase.
Moreover, a power loss by a current detection resistance becomes large to an unignorable extent as the output current of the switching regulator becomes large. As a technique to solve such a problem, a control unit for the switching power supply which can execute the current mode control without actually detecting the inductor current and a switching power supply are known (for example, refer to Patent Literature 1).
FIG. 1 is a circuit block diagram showing a configuration of the control unit for a switching power supply disclosed in Patent Literature 1. In the technique disclosed in Patent Literature 1, the current mode control is realized by the control unit for the switching power supply without detecting the inductor current. Referring to FIG. 1, the control unit for the switching power supply is provided with an analog-to-digital (A/D) converter 111 which A/D converts the output voltage Vo of the switching power supply, and a controller IC 118. The controller-IC 118 is provided with a difference circuit 112 which subtracts the output signal of the A/D converter 111 from a reference digital signal Vref and outputs an error signal, a gain circuit 170 which has a gain of the factor of G, a PWM signal generating section 128 which generates a PWM signal PS, and a feed-back circuit 129 which provides a current estimation function.
The PWM signal generating section 128 is provided with a voltage comparing circuit 124, an R-S flip-flop 127 and an AND circuit 172. A feed-back circuit 129 which provides the current estimation function is provided with an up and down counter 173, a low pass filter 174, a reset generating circuit 175 and a difference circuit 176. The difference circuit 176 subtracts the output signal DC of the low pass filter 174 from the output signal PC of the up and down counter 173.
In the control unit for the switching power supply disclosed in Patent Literature 1, a drive pulse PS is fed back to the current estimation function, and the inductor current in the switching power supply is estimated based on the drive pulse PS by using the current estimation function, to generate an estimated current signal PC. Moreover, in the control unit for the switching power supply disclosed in Patent Literature 1, by a DC component removal function, a DC component DC is extracted from the estimated current signal PC and the DC component DC is removed from the estimated current signal PC by the difference circuit 176.
The controller IC 118 is configured from a digital circuit operating with a master clock having the frequency of 10 MHz to 100 MHz and controls the switching power supply. The error signal as a difference between the A/D-converted digital output voltage Vo from the A/D converter 111 and the reference voltage Vref is amplified to the gain of G times according to P (proportion) control to generate a control signal CS.
Also, the PWM signal PS is negatively fed back and the estimated inductor current signal PC is generated by the feedback circuit 129.
FIG. 2 shows timing charts in the operation of the controller IC 118 disclosed in Patent Literature 1. The PWM signal PS is supplied to the up and down counter 173. The up and down counter 173 counts up according to the coefficient of “a” when the signal level of the PWM signal PS is H (High), and counts down according to the coefficient of “b” at the time of L (Low). These coefficients show increment/decrement rates of the inductor current. By the low pass filter 174 and the reset generating circuit 175, an error component (DC component) DC superimposed on the estimated inductor current signal PC is extracted and the estimated inductor current signal PC′ is generated by correcting the error component DC.
Moreover, the control signal CS and the estimated inductor current signal PC′ are compared by the voltage comparing circuit 124 and the PWM signal PS is generated based on the comparison result CO.
Patent Literature 2 discloses a digital PWM wave generating apparatus which makes the frequency of the PWM signal constant surely. The digital PWM wave generating apparatus is provided with a digital triangle wave generating section, a digital threshold value outputting section and a digital comparator. The digital triangle wave generating section outputs a digital triangle wave. The digital threshold value outputting section outputs a digital threshold value. The digital comparator compares the digital triangle wave and the digital threshold value and outputs the PWM signal. The time change rate of the digital threshold value when the digital threshold value outputting section changes the digital threshold value from a first constant value to a second constant value is set to a value equal to or smaller than a time change rate of the digital triangle wave.
Citation List
[Patent Literature 1]: JP 2004-282961A
[Patent Literature 2]: JP 2001-111396A